1. Field of the Invention
This invention relates to a monochromatic image display system, and more particularly to multiplication of the number of display tones. This invention further relates to a medical flat display panel for monochromatic image display.
2. Description of the Related Art
As an image display system for displaying a monochromatic image, there has been known one using a CRT (cathode-ray tube). Further there has been in wide use a flat panel display (FPD) using a liquid crystal panel. Owing to the fact that the flat panel display requires less space, is smaller in weight and consumes less power than the cathode-ray tube, it is expected that the flat panel display will spread more widely.
As a method of expressing tones of a monochromatic image in the flat panel display, there has been known a method in which tones are expressed according to a luminance signal (will be referred to as “intensity modulation” hereinbelow). In a system where a liquid crystal panel is employed as a display device, there has been known a method in which tones are expressed by changing the duration of display per unit time by time division drive which controls times for which switches are kept on or off per unit time, e.g., pulse width gradation control or frame thinning control (will be referred to as “time modulation” hereinbelow). For example, see “Denshi Gijutsu”, extra edition, May (Vol. 32, No. 7), pp. 110 to 121. Further there has been proposed a method in which the number of tones of a monochromatic image which can be expressed is multiplied by a combination of the time modulation and the intensity modulation.
However this approach is disadvantageous in that the number of tones cannot be unlimitedly multiplied since the number of division of the unit time is limited due to limitation of the response speed of the liquid crystal and it is difficult to display a monochromatic image which is rich in expression.
Further, in the medical field, there have been put into practice various diagnostic image taking means using X-rays or the like such as an X-ray apparatus, a CR (computed radiography) apparatus and the like.
The medical image information obtained by such diagnostic image taking means is subjected to a desired image processing such as a frequency processing, a tone processing and the like and is converted to an TV image signal, for instance, on a NTSC system. Then the TV image signal is reproduced as a visible image on a soft copy system such as a CRT display or as a visible image recorded on a photosensitive material (photographic film) by a LP (laser printer) or the like. The visible image recorded on a photographic film is generally fixed on a view box and submitted to observation. The medical image information obtained by the diagnostic image taking means is thus used in inspecting existence of a lesion or disease and/or the condition thereof. As the soft copy system, though CRT displays have been prevailing in the past,.flat panel displays using a liquid crystal panel, an organic EL panel or the like recently have come to be in wide use and it is expected that the flat panel display will spread more widely in the medical field owing to the fact that the flat panel display requires less space, is smaller in weight and consumes less power than the cathode-ray tube.
The CR (computed radiography) apparatus is a radiation image recording and read-out apparatus which records a radiation image of an object by use of “stimulable phosphor” and has spread widely recently. That is, certain kinds of phosphors, when exposed to a radiation, stores therein a part of the energy of the radiation and emit light in proportion to the stored energy of the radiation when exposed to stimulating rays such as visible light, infrared rays or the like. A phosphor exhibiting such properties is referred to as a stimulable phosphor. In the CR apparatus, the stimulable phosphor layer in the form of a sheet is exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon and is then exposed to stimulating light which cause the stimulable phosphor layer to emit light in proportion to the stored radiation energy, and the light emitted from the stimulable phosphor layer is photoelectrically detected, thereby obtaining an electric image signal representing the radiation image of the object. See, for instance, Japanese Unexamined Patent Publication No. 62(1987)-18536.
In the case where a medical image recorded on a photographic film is observed on a view box as described above, the medical image is observed as a monochromatic image of blue base if the film is of blue base. Since, in the medical field, X-ray films have been of blue base for a long time, doctors and/or radiographers have been accustomed to making a diagnosis on the blue-base image. Accordingly, there has been a demand that medical images should be displayed as a blue-base monochromatic image on a soft copy system as in the case where the medical images are recorded on photographic film and observed on a view box.
However, in flat panel displays such as of liquid crystal, though some of them can make a display in a predetermined monochromatic tone, they are for a green-base or amber-base monochromatic display and not for a blue-base monochromatic display. Accordingly, in order to make a blue-base monochromatic display on a soft copy system, there is nothing for it but to use a display system using color display devices for red, green and blue image signals and cause the display system to make a blue-base monochromatic display by adjusting the signal levels to the respective display devices.
In the display system using color display devices, in order to have the color display devices matched with a black and white display device, the display outputs of R, G and B are set in the ratios of about R:G:B=0.3:0.6:0.1 so that the color display devices are substantially the same as that of the black and white display device, and the mixing value Y (=R+G+B) is taken as the luminance level. In this case, when the R-signal level, the G-signal level and the B-signal level are all at 100%, that is, white level, the display luminance level is at 100%. For example, in the case of a CRT display system, when the display luminance level is at 100%, the maximum luminance is normally about 100 to 200 cd/m2. The maximum luminance of a liquid crystal panel or an organic EL panel is normally lower than that of the CRT display system.
Therefore, when the levels of the R-signal and the G-signal are lowered in a display system using color display devices in order to make a blue-base monochromatic display, the total luminance is lowered greatly below that which can be obtained when the medical image is recorded on a photographic film and observed on a view box, i.e., 5000 to 6000 cd/m2.
From the viewpoint of brightness discriminating ability, it is known that the brightness discriminating ability is maximized when the luminance level is in the range of 50 to 500 cd/m2. When the luminance is 100 to 200 cd/m2 at the maximum, the expression range at film density 1 (minus one figure of the maximum luminance) which is often used in observing medical images is only about 10 to 20 cd/m2, which gives rise to a problem from the viewpoint of brightness discriminating ability. From the viewpoint of sight (resolution), it is said that at least 10 cd/m2 of mean luminance is necessary to keep eyesight of not lower than 1.0. When the expression range is only about 10 to 20 cd/m2, there is little allowance also from the viewpoint of sight, which gives rise to a problem.
In other words, since, in the medical fields, the expression range corresponding to film density 1 is often used, it is preferred that the maximum luminance range be 500 to 5000 cd/m2 so that the expression range corresponding to film density 1 becomes 50 to 500 cd/m2 where the brightness discriminating ability is optimized.
Further each of the R-, G- and B-signals is generally of 8-bit and accordingly, when the monochromatic tones are expressed by mixing these signals, the number of the tones is 256, which is insufficient as a display system of medical images.